1. Field of the Invention
This invention relates to an apparatus for measuring the water vapor permeability through sheet materials such as films, foils, paper or cardboard, under adjustable constant measuring conditions.
2. Description of the Prior Art
For testing the water vapor permeability of plastic foils, paper or cardboard, the German Standard DIN 53122 stipulates stretching the sample to be tested in a water vapor-tight manner over a dish filled with an absorption medium, providing a stream of humid air containing a known percentage of relative humidity over the sample and determining the weight increase of this dish by weighing.
The disadvantage of this method is that the weight increase is based on weighing the dish, exactly to the nearest milligram, which itself weighs some hundreds of grams. Not only is such weighing cumbersome, because the dish must be taken out of the apparatus, but it also leads to measurement errors due to the large difference between the weight of the dish, typically on the order of 500 grams, and the accuracy of the required measurement, namely to the nearest milligram, or preferably, the nearest tenth of a milligram.